The present invention relates to methods for determining the porosity of earth formations surrounding a borehole by use of known neutron logging methods, but without any prior knowledge of formation lithology.
Numerous logging methods are used in efforts to determine the characteristics of the earth formations surrounding boreholes which have been drilled for production of oil, gas, or other minerals. For example, electrical resistance is measured in an effort to distinguish between the types of liquids in the formation pore space, for example, to distinguish between water and either oil or gas. Other logging techniques use various neutron measurements to distinguish between various types of minerals which make up the lithology of the formations surrounding boreholes. Sonic logs are used to determine formation acoustic velocity and to possibly identify fractures. However, all of these logging techniques share a common problem. In general, none of the measurements is a direct measurement of the characteristic being investigated. In nearly every case, the actual logging tool measurements or outputs must be analyzed in order to make an estimate of the characteristics, such as mineral type or porosity which is being studied.
The determination of porosity from the various log measurements has been one of the most difficult problems faced by log analysts. Many models have been introduced which combine more than one log to form a series of linear or sometimes nonlinear equations where the unknowns are porosity and the mineral volumes in the reservoir. The number of equations that can be used depends on the number of logs that have been run, so that the more logs the better the estimate. For the all of the prior methods to be accurate, a great amount of work should be given to identifying the formation lithology, that is, the mineral content of the matrix rock forming the formation. If this is not measured directly from core samples or drill cuttings, then it must be estimated. The accuracy of the porosity estimations is of course a function of the accuracy of such estimates.